1. Field of the Invention
The present invention relates to a switching power supply for supplying power to a device.
2. Description of the Related Art
FIG. 9A illustrates a configuration of a conventional switching power supply. In FIG. 9A, an alternating current (AC) input from a commercial alternating current power supply is rectified by a diode bridge configured from D1, D2, D3, and D4 via switches SW1 and SW2, and is smoothed by a primary electrolytic capacitor C1. If the AC voltage is AC 120 Vrms, a C1 terminal voltage Vh is about DC 170 V. Vh is supplied to a field-effect transistor (FET) 1, which is a switching element, via a primary winding of a trans T1. On the other hand, Vh is also supplied to a pulse-width modulation (PWM) controller CONT 1. If Vh is equal to or more than a prescribed value (in this example, DC 50 V), CONT 1 supplies a pulse signal OUT to a gate terminal of the FET 1. Based on this pulse signal OUT, the FET 1 starts switching. When the FET 1 starts switching, a pulse voltage is induced in a secondary winding coil of the trans T1. This pulse voltage is rectified by a secondary rectification diode D5, and is smoothed by a secondary smoothing capacitor C2. Thus, a C2 terminal voltage Vout is a roughly constant voltage (in this example, DC 24 V).
Next, the operation of this switching power supply will be described. The output voltage Vout of the switching power supply is supplied to the respective modules mounted in the device. The respective modules in the device are, for example, a control unit CONT 2 configured from a central processing unit (CPU) and an application-specific integrated circuit (ASIC) for controlling operation of the device, and an actuator M that is an active device such as a motor or a solenoid. A control signal MTR is supplied from the control unit CONT 2 to the actuator M. The control unit CONT 2 drives the actuator M by setting the MTR signal to a HIGH level (hereinafter referred to as H level). Conversely, the control unit CONT 2 stops the actuator M by setting the MTR signal to a LOW level (hereinafter referred to as L level). The configuration of such a switching power supply is discussed in Japanese Patent Application Laid-Open No. 10-66334.
In the above switching power supply, there are the below-described problems when the switches SW1 and SW2 have been turned OFF. Operation when the switches SW1 and SW2 have been turned OFF will be described using FIG. 9B. In FIG. 9B, at time t1, when the switches SW1 and SW2 are turned OFF, the power supply from the commercial alternating current power AC (alternating voltage) is cut. After time t1, the switching power supply operates based on the power (charge) that has accumulated in the primary electrolytic capacitor C1. The terminal voltage Vh of the capacitor C1 gradually decreases from DC 170 V. At time t2, at which this Vh has dropped below the prescribed value (in this example, DC 50 V), a PWM controller CONT 1 stops output of an OUT signal. Consequently, the output voltage Vout of the switching power supply decreases, and the power supply (device) comes into a turned OFF state. Therefore, even though the switches SW1 and SW2 are OFF, during the period t2-t1, which is from the time t1 to the time t2, an output voltage Vout of the switching power supply is being produced. Further, the smaller the output power of the switching power supply is the longer the period t2-t1 becomes. Specifically, the smaller the amount of power consumed by the control unit CONT 2 and the actuator M, the longer the period t2-t1 becomes.
However, there is a need for a greater reduction in power consumption when a device is on standby. The amount of power that is consumed during standby is extremely small, as various measures are taken, such as stopping the actuator M and setting the control unit CONT 2 to a power-saving mode. Therefore, the above-described period t2-t1 becomes very long. For example, in some cases this period t2-t1 is several dozen seconds.
For example, if the device is a personal computer (PC) or some other information device, the user may want to increase the memory of the PC. In increasing memory, it is desirable that Vout has become zero to ensure the reliability of the memory and the information device itself. This is because if the user tries to add memory before Vout has reached zero, in a state in which charge still remains (conducting state) (also called hot-swapping), the memory can be damaged. Therefore, it is necessary to avoid such hot-swapping.
The memory could be added after the switches SW1 and SW2 have been turned OFF. However, Vout does not immediately decrease even if the switches SW1 and SW2 are turned OFF. During the period t2-t1 until Vout decreases, the user cannot add the memory, and the user has to wait until Vout reaches zero (until the charge stored in the primary electrolytic capacitor C1 reaches zero). Under these circumstances, usability is poor, since the user has to wait when adding memory.
This wait time is not limited to memory expansion. For example, such a wait time is a factor which deteriorates usability in various situations, such as during equipment maintenance performed by a repairman and when a device has to be restarted.
To resolve this problem, the device illustrated in FIG. 10 has been proposed. In FIG. 10, an alternating voltage from a commercial alternating current power AC is supplied to a diode D6, a resistor R7, and the light-emitting diode (LED) side of a photocoupler PC-A via a contact point of switches SW1 and SW2. On the PC-A phototransistor side, power is supplied as an ANS signal to the CONT 2. The operation waveform of this circuit is illustrated in FIG. 11. In FIG. 11, if the alternating voltage from the commercial alternating current power AC has a positive polarity, current flows to the LED side of the photocoupler PC-A, and the ANS signal is at an L level. Conversely, if the alternating voltage from the commercial alternating current power AC has a negative polarity, current does not flow to the LED side of the photocoupler PC-A, and the ANS signal is at an H level. More specifically, this ANS signal is a pulse signal that is synchronized with the frequency of the alternating voltage from the commercial alternating current power AC. This circuit configured from the diode D6, the resistor R7, and the photocoupler PC-A is also called a frequency detection circuit.
If the switches SW1 and SW2 are turned OFF at time t1 by this frequency detection circuit, and the alternating voltage supply from the commercial alternating current power AC to the device is cut, the ANS signal is fixed at the H level. If the ANS signal is output at the H level for a prescribed time or more, the control unit CONT 2 determines that the supply of alternating voltage from the commercial alternating current power AC has been stopped. However, in this method for detecting stopping of the alternating voltage supply with the frequency detection circuit (also referred to as AC-OFF detection), during the positive polarity half-wave of the alternating voltage from the commercial alternating current power AC, the LED current of the photocoupler PC-At is constantly flowing. This means that the photocoupler PC-A is consuming power even during device standby. This leads to the problem that power consumption cannot be sufficiently decreased in a power-saving operation state, such as during device standby.